Identification and evaluation of internal leakages of a BFB Boiler integrated within a pulp and paper mill.

Abstract: Alternative fuels like biomass have become really popular in the last decades as a substitute to fossil fuels. One of the most used technologies in Sweden for the obtention of the energy from the biomass is its direct combustion in a boiler. Bomhus Energi is a company with the purpose of creating steam and district heating for Billerudkorsnäs pulp and paper mill in Gävle and district heating for the city by operating a biomass BFB boiler. Despite being a quite new boiler, there are many sources of errors, losses and unnecessary costs. Between huge number of different losses that can happen in this kind of industrial boilers, the concern about internal leakages is not usually popular among plant designers and operators. This often leads to forget about them or not giving the importance that they could have.This study consists on, firstly, an analysis of different boiler equipment that have potential possibilities of internal leakages by mass and energy balances and by the tracking of possible mass losses. The second point of this thesis is to evaluate the cost of internal leakages that could have happen before, in order to be aware of how important they are. Additionally, measures are proposed in order to avoid or reduce the duration of the internal leakages, where the most common problem is the ignorance of their existence. This study focuses partially on the valve condition and maintenance. It is highly important to carry out valve maintenance procedures at least once per year during the general stop of the plant. Checking and verifying valve perfect conditions, can avoid a waste of a huge quantity of money just by replacing some internal elements that are possibly damaged due to the extreme working conditions. This small damages in valve can lead to a non-proper water tightening, which will be increasing its leakage over time. In the present paper, possible internal leakages through the valves belonging to the feedwater, steam drum, preheater and pressure vessel in general have been the principal aim. The key of this study was to take into account that biggest part of the draining system and valves that are supposed to be closed end in the Bottom blowdown tank. By then a deep study was done regarding this tank. The results show that there is a clear relationship between mass that is getting loss from feedwater tank and pressure vessel and the necessary cooling flow in the bottom blowdown tank. This means that if the cooling flow increases at the same rate as a possible leakage in mass and energy balance, there is an internal leakage somewhere in the system. The author proposes add an alarm to the DCS system in order to alert the plant operations of possible internal leakages. On the other hand, this paper also recommends to carry out a general valve maintenance per year and check which of them could be leaking, a general stop is the perfect time for carrying it out.In conclusion, the study finds that internal leakages can be even automatically detected if the system is provided with the necessary tools for it. The study concludes that internal leakages are not impossible to detect and their cost is non-negligible: the latest two internal leakages in the boiler, happened in the last two years, were from the feedwater draining system and from the steam drum heating loop with a total cost of 200,000 SEK (4240 SEK/day during 47 days) and 263,000 SEK (2120 SEK/day during 124 days) respectively. Additionally, days after the study, the plant general stop was carried out, finding that 12 valves were leaking due to internal damage. The cost of repairing the broken or damaged elements were almost negligible compared with the expected savings estimated in 2 Million SEK per year.